A fuel tank for a motor vehicle such as an automobile or motorcycle is commonly used for storage of gasoline. If the airtightness of the fuel tank is poor, gasoline, which has been vaporized in the tank, diffuses to the open air. Diffusion of gasoline is one of the causes which adversely affects the global environment, which is especially valued recently. For instance, a conventional fuel tank made of synthetic resin does not assure sufficient hermetic structure against diffusion of vaporized gasoline. Another type of a fuel tank is also known, which is made of a surface-finished steel sheet such as a Zn or Al-coated steel sheet. However, the coating layer is often peeled off or cracked during reforming of the surface-finished steel sheet to a product shape. Such defects as cracks or flaking act as starting points for corroding reaction, when the fuel tank is exposed to a corrosive atmosphere containing organic acids generated by degradation of gasoline. Pitting corrosion causes formation of holes opened through a wall of the fuel tank, and significantly worsens hermetic structure of the fuel tank.
Applicability of stainless steel, as representative corrosion-resistant material, has been researched and examined for fabrication of a fuel tank in order to maintain hermetic structure over a long term. But, formability of stainless steel is poor compared with cold reduced carbon steel, so that work-induced cracks often occur when it is shaped to a fuel tank by a complicated process accompanied with deep-drawing and bulging. Therefore, it is difficult to manufacture fuel tanks from stainless steel sheets by an economic and stable pressing process. Even when a stainless steel sheet is press-formed to a desired shape, a fuel tank is not of a sufficient hermetic structure to inhibit diffusion of vaporized gasoline, due to the presence of microcracks. Moreover, work-induced defects act as starting points for corroding reaction, and causes growth of pitting to an extent passing through a wall especially at an inner surface of a fuel tank exposed to depleted gasoline.
However, stainless steel, which is bestowed with formability necessary for fabrication of a fuel tank and also good of corrosion resistance, has not been proposed so far.